How (and Why) to Measure Return Air Grille Pressure Drop

You know the sound when the fan comes on and the filter in the return grille smacks the back of the filter grille housing? Is that good or bad for your system? How much is that return air filter grille restricting system airflow? How is it affecting airflow? Is there a way to know for sure?

The reason an increasing number of us are taking air balancing measurements is to figure out how well our systems are or aren’t working. There’s an unseen world in our systems of airflow, pressures, temperatures, BTUs, velocities, and Watts that make a huge difference in comfort and efficiency.

Testing is the way we “see” what’s going on in there, so we can then make needed improvements in the system. We've done this for decades with electrical and mechanical testing, but the new emerging frontier is to understand and improve airflow and BTU delivery. These can now be measured and verified.

How Much Pressure Drop?

Before we learn to measure filter grille pressure drop, let’s figure out what our measurements are telling us. The rule of thumb for return air filters and filter grille pressure drop is that ideally it should not exceed 25% of the rated maximum total external static pressure of the system fan.

Here’s how to determine that number. First, go to the air handler nameplate. Read it until you find the Maximum Total External Static Pressure. Most of the time it will be 0.50-in. w.c. Typically, this means that as long as the total system pressure remains below the 0.50-in., the fan will deliver at least 400 cfm/ton. That’s the required airflow needed to allow your cooling system to operate efficiently and for the system to deliver the goods.

Multiply the rated Total External Static Pressure by the 25% rule of thumb. So, 0.50-in. times 25% equals an ideal filter grille static pressure drop of 0.125-in. A 1.00-in. variable speed fan allows the filter pressure should ideally be less than 0.25-in. (1.00-in. x 25%). When calculating pressure drop over the filter only, you’re looking for 20%, but with the grille included, use the 25% rule.

Now, rules of thumb are called rules of thumb because there’s more to it than the rule is saying. Here’s the rest of the story.

There are times when the filter grille pressure drop may exceed 30% or more of rated fan total external static pressure. This is when the pressure drop of the coil, and duct system are low enough that the system’s total external static pressure remains below the maximum including the overly restrictive filter. As long as the system’s total external static pressure remains less than the rated maximum, you may use a more restrictive filter.

The primary reason to use filter grilles is to install more filters into the system. As you increase filter surface area, the pressure drop, or restriction caused by the filters, decreases. This happens because you’re now running less pounds of air per square foot over the filter. One caution: if you’re going to filter the air at return air grilles, make sure your return ducts are generously sized and are extremely tight, or particulates will load up your fan and coil before you know it.

Test Time Now that you know what to look for in your filter grille pressure drop measurement, it’s time to test. Since a picture’s worth a thousand words, take a look.

Now that you’ve got the picture, let’s look at the steps to the test. First, turn the fan on. Normally we test in cooling mode because it’s the toughest condition the system will be operating under.

Set the stat to cooling mode, fan on, and 55F degrees. Run the system for 10 minutes or so to allow the coil to get wet. Then take the test.

Select a good manometer. If it’s a digital gauge, be sure the range is appropriate. Zero to 5-in. for a cheap one, and 0 to 10-in. for a good quality manometer. Since real air conditioning men (and women) measure pressure in inches of water column, get a manometer that measures in inches, not Pascals. If you’re using an analog manometer, it must be adjusted to zero before testing and be held level for testing.

Attach the tube or pressure hose to the negative port on the manometer. This would be the right, or bottom port on most manometers. Leave the other or positive port opened to the room pressure. If your manometer only has one port, donate it to charity and get one suited to air conditioning testing that has two probes as soon as you can.

Attach a right angle static pressure tip (like the one shown in the drawing here) to the end of the tube. Insert it through the filter grille and the filter with a firm thrust of your predominate hand.

The pressure behind the filter is collected by the static pressure tip. The pressure in the room is collected by the open port on your manometer. The difference, or pressure drop over the filter grille, appears on the manometer.

If you’re using a analog gauge and the needle falls left to the pen, simply change the hose to the other port on the manometer. Do it calmly, no one knows you made a mistake.

Now that you have your reading, compare the reading to the ideal reading that you calculated. If the pressure reading is less than the ideal pressure drop, congratulations. If not, you have the opportunity to improve system performance.

Filter Corrections

You can make needed corrections by replacing the filter with a less restrictive filter that your fan can actually afford to have in the system. If increased filtration is needed for a particular reason, you will need to add additional filter surface area. This can be done by installing more filters in the system by using additional return air filter grilles, or by building a multiple-filter housing in the return side of the system. You may also investigate the HEPA filters units that come complete with their own fan and have little or no pressure effect on your systems.

Now the fun begins. Go out and test a few. Like most new things that we do, it will feel a little awkward the first few times, but you’ll get the hang of it. Don’t be discouraged, many filters have twice the pressure drop the fan can afford, but problems in our trade are opportunities.

Add this test and static pressure testing to all the systems that you encounter and double your effectiveness by pulling your head out of the box and expanding your horizons by measuring how a system is performing, not just checking if it’s working or not.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a free static pressure test procedure, contact Doc robf@nationalcomfortinstitute.com or call him at 800/633-7058. Go to NCI’s website at www.nationalcomfortinstitute.com for free information, technical articles and downloads.

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